A prior art liquid crystal spatial light modulator (LCSLM) may be formed of liquid crystal (e.g., nematic) material sandwiched between two conductive films (at least one of which is transparent). If one of the conductive films is pixilated and electrically addressable, an electric field may be individually generated through the liquid crystal material at any selected pixel, to change the effective refractive index of the liquid crystal in that pixel. In this manner, selective phase change may be imparted to a wavefront transmitted through the liquid crystal material according to the induced phase retardation caused by the array of pixels. The amount of phase change is, approximately, linearly proportional to the applied voltage, typically within 30˜50% of the total possible phase change region.
For the prior art LCSLM, voltage applied to a pixel is constant within the pixel; therefore, the induced phase change within a pixel is also constant. Accordingly, each pixel is decoupled from neighboring pixels and the wavefront generated from the LCSLM exhibits characteristics of “piston” type (stepwise) error. Still, absolute calibration is easy to implement for these LCSLM devices and they are suitable for feed-forward adaptive optics applications.
But use of the prior art LCSLM to simulate a continuous wavefront generates small but non-negligible wavefront errors. To achieve Strehl ratios above 0.8, the phase gradient in adjacent pixels must be small. To reduce the wavefront errors, the prior art LCSLM must also utilize a high number of pixels in order to correct wavefront distortion with high dynamic range or the high amplitude of phase distortions. Increasing the number of pixels increases the size, weight and power of the LCSLM controller, further reducing optical efficiency because of reduced pixel fill factor and thereby limiting the LCSLM's laser power handling capability.
Piston type wavefront is also not commonly detected. For example, a frequently used Shack-Hartmann sensor detects wavefront by sensing wavefront slope and then reconstructing the wavefront; if it is used to detect the piston type wavefront, a precise spatial registration between SLM pixels and the lenslet array in the sensor is required. Interferometry may also be used to detect phase difference between a reference beam and the wavefront to be measured; but sophisticated optics is required to generate a local oscillator.
U.S. Pat. No. 6,458,673, U.S. Pat. No. 6,761,981, U.S. Pat. No. 6,832,028 and U.S. Patent Publication Nos. 2004/0067013 and 2002/0187283 are incorporated herein by reference.